Electric vehicles need large electric power storage devices. The large electric power storage devices, which are presently being developed, are mainly secondary batteries (also known as storage batteries). These secondary batteries have the great disadvantage of having a short cycle life. In particular, it is generally said that the nominal life of ordinary secondary batteries expires after about 300 to 500 discharges are completed, although the number of the discharges differ, depending of the kind, the rated values, and the depth of discharge. With respect to individual secondary batteries, lead-acid batteries are most often used. The life of lead-acid batteries shortens drastically when the depth of discharge exceeds 60-70%. If nickel-cadmium batteries are recharged before completion of discharge, a significant loss of storage capacity takes place.
An electric double layer capacitor uses activated carbon as its electrode material. Activated carbon is electrochemically inactive and has a large specific surface. The activated carbon is combined with an electrolyte to form a large electric double layer capacitance. The electric double layer capacitor has attracted attention as a power source for an electric vehicle (electric vehicles are an alternative to vehicles with internal combustion engines because electric vehicles produce no harmful exhaust gas). Electric double layer capacitors can be charged in shorter times and have longer cycle lives than secondary batteries. A cycle life of more than 10,000 recharges has been estimated from the experiment carried out by the inventors.
A secondary battery has the advantage that its working voltage does not drop greatly even after the battery has been discharged. Even if the voltage drops, only a slight voltage drop is observed under unloaded condition. For example, in the case of a lead-acid battery, the terminal voltage in a fully charged state is 2.2 volts. This voltage drops only to about 1.8 volts except during charging and in an immediately subsequent transient state. This means the battery is a constant-voltage device which can be charged with little loss by the use of a voltage regulated source. Methods and apparatus for charging secondary batteries are known.
However, techniques for efficiently charging an electric double layer capacitor of large capacitance in a short time have not been yet established. One conceivable method of charging an electric double layer capacitor is to use a voltage regulated source in the same way as in the above-described case of secondary batteries. This is discussed further below.
Energy Ec stored in a capacitor having terminals across which a voltage V is developed is given by Ec=CV.sup.2 /2, where C is the capacitance of the capacitor. The voltage V across the terminals drops as the capacitor is discharged. Conversely, if the capacitor is charged under a fully discharged state, the voltage across the terminals gradually rises from zero. That is, the capacitor is not a constant-voltage device.
If the capacitor, which is not a constant-voltage device, is charged with a voltage regulated source producing an output voltage V, for example, then the output voltage V is principally applied to a resistive component existing in the charging circuit when a charging operation is started, i.e., when the voltage across the terminals is null. The electric power is wastefully consumed by this resistive component. As the charging progresses and the voltage across the terminals increases, the electric power consumed by the resistive component (other than the capacitor) decreases and becomes zero when the voltage across the terminals reaches V.
The total energy Ep supplied from the power supply until the charging operation is completed is equal to the product of the integrated current flowing to the capacitor, or the total charge, and the voltage V. That is, Ep=QV=CV.sup.2.
The charging efficiency, which is defined as the ratio of the energy Ec to the energy Ep, is calculated at 50%. Fifty percent of electric power which is not stored in the capacitor is consumed by the resistive component existing in the charging circuit as described above.
This means that if the capacitor is charged from a voltage regulated source, the maximum charging efficiency attainable is only 50%. As an example, as shown in FIG. 1, if a dry battery voltage source VS is connected in parallel with an electric double layer capacitor CB having an internal resistance RB and is electrically charged, then only half of the energy released from the dry battery voltage source VS can be stored in the capacitor.
It is an object of the present invention to provide a method and apparatus for charging an electric double layer capacitor efficiently and quickly.
It is another object of the invention to provide a method for charging an electric double layer capacitor up to its rated level in a short time.
It is a further object of the invention to provide a method for charging an electric double layer capacitor efficiently and quickly if the capacitor is charged from a solar battery.